Disk drive devices using various kinds of disks, such as an optical disk, a magneto-optical disk, and a flexible magnetic disk, have been known in the art. In particular, a hard disk drive (HDD) has been widely used as a storage device of a computer and has been, one of indispensable disk drive devices for current computer systems. Moreover, the HDD has found widespread application such as a removable memory used in a moving image recording/reproducing apparatus, a car navigation system, a cellular phone, or a digital camera, as well as the computer due to its outstanding characteristics.
Magnetic disks used in the HDD have a plurality of data tracks formed concentrically and a plurality of servo data sectors arranged discretely in the circumferential direction. User data is recorded in data sectors as units and the data sectors are recorded between respective servo sectors. A swinging actuator moves a head slider over a rotating magnetic disk. A head element portion of the head slider accesses a desired data sector according to positional information indicated by a servo sector so that writing and reading to and from data sector can be performed.
Servo sectors are preliminarily recorded on a magnetic disk in manufacturing an HDD. Typically, mechanical or electrical control of the head and the actuator of the HDD after mounting the magnetic disk on the HDD record the servo sectors on the magnetic disk. Due to an eccentricity of the magnetic disk or other factors such as external vibration, the recorded servo data may have deflections from an ideal annular track generated during writing the servo data or after recording the servo data. Hence, positional data which the head element portion reads out from a servo sector contain a component called repeatable run-out (RRO).
Although there is no problem if the RRO is small, if the RRO gets large, an accurate head positioning to a target position becomes difficult even though the head positioning is performed according to the read servo sector. However, there is a problem in memory capacity if data for compensating the RRO are stored in a memory with respect to all servo sectors.
Therefore, an approach has been proposed to reduce the RRO: data for compensating the RRO are preliminarily recorded in the magnetic disk (refer to Japanese Patent Publication No. 2003-53145 “Patent Document 1”, for example). The head element portion reads out RRO compensating data in addition to usual positional data. The HDD controls the actuator so that the usual positional data read out by the head element portion gets close to the target position. On this occasion, it controls the actuator based on compensation by RRO compensation data read out by the head element portion. Typically, the RRO compensation data are recorded next to the positional data continuously in each servo sector.
The head element portion has a read element and a write element and there is an offset between the read element and the write element in the radial direction. This offset is called read/write offset. The read/write offset changes according to the radial position due to a skew. Therefore, there exists a discrepancy in the target radial position of the read element in between a writing process and a reading process. Therefore, in order to perform RRO compensation in both the reading and writing processes, it is required that RRO compensation data are recorded in each read element position. Recording the RRO compensation data onto the magnetic disk means a decrease of the region for recording user data.
One possible approach to this issue is a control that RRO compensation is performed in the writing process, and is not performed in the reading process. In the control, it is required only to record the RRO compensation data only for the writing process so that the decrease of the user data region is suppressed. However, in such control, head positioning in the reading process will be a problem.
Or, as disclosed in a Patent Document 1, RRO compensation in both reading and writing processes can be realized by recording RRO compensation data for a writing and a reading processes alternately in each servo sector discretely recorded in the circumferential direction. This means, however, that the RRO compensation in each process is performed with alternate servo sectors. Therefore, sufficient RRO compensation cannot be performed especially in the writing process so that problems such as off-track writing and squeezes between adjacent tracks may occur.